The objective of this project is to elucidate the biochemical basis of the immunological cross-reactivity between the mouse mammary tumors and human breast carcinomas. Results of our previous study indicate that antibodies in sera of some patients with breast cancer react with antigens in BALB/cf C3H mouse mammary tumors and that this reactivity could be absorbed with mouse mammary tumor virus (MMTV). Specificity of the reaction of human antibodies with MMTV was verified by absorption of human sera with a large number of glycoproteins, tissues and viruses other than MMTV. To identify components of MMTV reacting with human antibodies, the human sera will be precipitated with lysed MMTV and the immunoprecipitates examined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The polypeptides of MMTV will be isolated and used as inhibitors in the reaction of human antibodies with MMTV in an enzyme-linked immunoassay (ELISA). ELISA will also be used to detect MMTV-related antigens (or antibody to such antigens) in human mammary tumor tissues. Antisera to purified polypeptides of MMTV will be prepared in rabbits and immobilized on Sepharose 4B. Affinity chromatography will be used to isolate MMTV-related antigens from human mammary tumor tissues. Physico-chemical properties of isolated human MMTV-related antigens will be investigated.